OS X 10.8 "Mountain Lion" is a version of Mac OS X released in 2012. It introduced various revamps to the Lion user interface and fixed many of the paradigms that were found to be controversial in Lion. A few feature to OS X, Gatekeeper, prevents unsigned code from running on a Mac by default (in an attempt to prevent malware), although it can be disabled. A system-wide notification system has also been ported from iOS, known as Notification Center.

This version of Mac OS X is the first to drop support for 32-bit kernels and corresponding 32-bit hardware. The decision to drop 32-bit architectures was made late in the development process, as early betas shipped with a 32-bit kernel.

Software Update is also now part of the App Store in this version of OS X.